Heterogeneous network (HetNet) deployments have low power nodes or small cells located throughout a macro-cell layout. Cellular operators have shown significant interest in HetNet deployments as a mechanism to enhance system performance, either in expanded coverage, increased capacity of number of users or data throughput, or both. One HetNet deployment has one or more of the low power nodes using the same carrier frequency as the macro cell, which tends towards co-channel interference.
Document R1-110687 by Qualcomm, Inc. entitled “INTERFERENCE ISSUES IN HETEROGENEOUS NETWORKS FOR HSPA” [3GPP TSG RAN WG1 Meeting #64; Taipei, Taiwan; 21-25 Feb. 2011] presents such a co-channel deployment for study of the evolution of High Speed Packet Access (HSPA) in the Third Generation Partnership Project (3GPP). Specifically, introduction of the low power nodes to the macro cell brings challenges for reliability of the control channel (namely, the high speed dedicated physical control channel or HS-DPCCH) and also for interference management between the low power nodes (LPNs) and high power nodes (HPNs). Mobility in wideband code-division multiple access (WCDMA) systems, including HSPA, is typically handled by using power measurements of the downlink common pilot channel (CPICH) which the network node B broadcasts with constant power. The CPICH technique has worked well in the past but it is a downlink metric only; in a HetNet scenario there are more likely to be several possible handover candidates and so to choose the best one it is useful to know the uplink pathloss from the mobile unit (more generally, a user equipment or UE) to each node B. Using uplink pathloss as a handover criteria helps ensure that the node B having the lowest pathloss will be used as the serving cell for the enhanced data channel (E-DCH).
In homogeneous network deployments where node B transmission powers are the same (e.g., all cells are macro cells), the CPICH measurements taken by the UEs are proportional to pathloss. This is not the case in HetNet deployments due to different transmission powers of the HPN node Bs versus the LPN node Bs.
One HetNet deployment option to avoid the reliability problem of the HS-DPCCH is to use what is known as a dedicated controller deployment in which the LPNs and the HPN are under control of different radio network controllers (RNCs). This deployment allows operators a more flexible choice of vendors, and could be a valuable solution when, for example, the RNC encounters capacity limits or limits to its HPN (node B) port connectivity. But the different RNCs would mean soft handover of the UEs would not be supported between an HPN and an LPN, and also the co-channel interference problem is likely to become more severe.
Consider again a more traditional HetNet deployment with the HPN and its nearby LPNs under the same RNC, which means soft handovers are supported. If there is an imbalance in the uplink (UL) versus downlink (DL) for E-DCH serving cell (i.e., having the best downlink with the strongest received signal but not the best uplink with the shortest pathloss), the UL scheduling information (SI) that includes the UE's uplink power headroom (UPH) and its data buffer status might not be received correctly by the serving E-DCH cell. This is because power control for the DPCCH is dominated by the best uplink of the radio link set (RLS) whereas the SI is only targeting the serving E-DCH cell which might have the worst uplink.
Document R1-110687 describes this also in its discussion at section 2 concerning the shared controller deployment scenario (same RNC for HPN and LPN); the strength of the signal received by each network node (HPN and LPN) does not depend on that node's downlink transmit power. Introduction of the LPNs potentially causes large uplink/downlink (UL/DL) imbalances in that cells other than the serving cell can receive much stronger UL signals from the UE than the serving cell receives. So for example a UE in soft handover from a serving HPN to a LPN can have a much better uplink with the LPN. The LPN can then give power commands to the UE such that the UE's transmit power is reduced to the extent that the still serving HPN gets only a very weak signal from it, resulting in the serving HPN being unable to reliably decode the UE's HS-DPCCH (which carries acknowledgements ACKs and negative ACKS (NACKs) as well as channel quality information CQI). Since the HPN is still the serving cell, this unreliable ACK/NACK decoding would result in excessive re-transmissions of data the UE may have already properly received, which degrades at least the DL performance. There are also interference issues for the co-channel HetNet deployment, for example when the interference victim LPN is not in the active set of the UE being served by a HPN and so the victim LPN has no opportunity to provide the UE with power control commands.
Distilling the above concerns, the problem then is how to address these large power imbalances that arise in co-channel HetNet deployments where the UL pathloss is not linearly analogous to the measured DL metric. The solution should be for a shared controller deployment where the HPN and the LPN are under the same RNC in order to support soft handover. The teachings below can be employed to address this problem, though this is but one non-limiting implementation thereof.